1. Field of the Invention
The present invention relates to a duplex printing device, including duplex stencil printing devices and the like, and more particularly to a duplex printing device which is capable of a duplex printing operation in which printing is performed on the front and reverse sides of a sheet of printing paper (to be referred to simply as “sheet” hereafter), and a simplex printing operation in which printing is performed on only one side of the sheet.
2. Description of the Background Art
Digital, thermal stencil printing is known conventionally as a simple printing method. In this stencil printing, a thermoplastic resin film typically having a thickness of approximately 1 to 2 μm is adhered to a porous support constituted by Japanese paper fiber, synthetic fiber, a mixture of Japanese paper fiber and synthetic fiber, or the like to form a stencil master (“master” hereafter) having a laminate structure. The thermoplastic resin film surface of the master is caused to contact heat-generating elements of a thermal head such that the master is heat-perforated and cut by an operation of the thermal head in a main scanning direction. The perforated master is then conveyed in a sub scanning direction (master conveyance direction) orthogonal to the main scanning direction by master conveyance means such as platen rollers, and thus wrapped around a rotatable print drum comprising on the outer peripheral portion thereof a porous, cylindrical plate cylinder constituted by resin or gauze mesh screen wound into a plurality of layers, for example. Ink is then supplied to the perforated master on the plate cylinder by an ink supplying member provided in the interior of the print drum, whereupon a sheet is pressed against the perforated master on the plate cylinder continuously by pressing means such as a press roller, impression cylinder, or in-press roller (to be represented by a press roller hereafter). As a result, ink is transferred onto the sheet from the porous part of the plate cylinder and the perforated part of the master such that printing is performed on the sheet.
Note that the term “print drum” sometimes refers simply to a plate cylinder, and sometimes to a plate cylinder provided on the outer peripheral portion of a print drum, but hereafter in this specification, usage of the term “print drum” is assumed to include the plate cylinder.
In recent years, duplex printing, in which printing is performed on the front and reverse sides of a sheet (also referred to as “both sides of the sheet” hereafter), has come to occupy the greater part of such stencil printing in addition to simplex printing, in which printing is performed on only one side of the sheet, with the aim of reducing sheet consumption and document storage space. A conventional duplex printing method uses a typical stencil printing device for performing simplex printing, such as that described above, to obtain duplex printed matter by passing a sheet stacked on a sheet feeding portion through a printing portion to print an image on one side (the front side) of the sheet, discharging and stacking the sheet onto a sheet discharge tray or the like, reversing the printed sheet, and again passing the sheet through the printing portion to print another image onto the other side (the reverse side) of the sheet. With this duplex printing method, however, the total printing time increases greatly since printing must be performed twice, or else time must be wasted waiting for the ink on one side of the printed sheet to dry, or set, sufficiently following simplex printing. Moreover, it is extremely troublesome to arrange simplex-printed sheets properly or to reset a simplex-printed sheet in the sheet feeding portion.
To improve this conventional duplex printing method which is based on a manual operation, development of duplex printing devices which can perform duplex printing automatically is flourishing. Many methods have been proposed as aspects of such a duplex printing device, but these can be divided mainly into the following six methods, which have the advantages and disadvantages described below.
(1) Two-Pass Duplex Printing Method in which a Sheet is Printed on One Side, Stocked, and Refed
A discharge unit comprising a discharge tray is constituted movably in relation to a device having a substantially identical constitution to that of a conventional simplex printing device. Furthermore, a sheet feed tray function is added to the discharge tray, and a novel reverse feed path and so on is added to form a so-called “automatic refeed mechanism”. By means of this constitution, a sheet stacked on a sheet feeding portion is passed through a printing portion and thereby subjected to surface printing, whereupon the surface-printed sheet is discharged onto the discharge tray, conveyed to the reverse feed path from the discharge tray, and thus fed automatically to a reprinting portion where reverse side printing is performed. Thus a duplex-printed sheet is obtained (see Japanese Patent Publication No. 2880052, for example). An advantage of this duplex printing method is that it can be used simply by making slight modifications to the constitution of a conventional simplex stencil printing device (annexing the automatic refeed mechanism described above thereto).
However, with the duplex printing method described in (1) above, firstly, perforation and printing must be performed twice, and therefore twice the perforation and printing time is required to obtain a duplex-printed sheet. Moreover, switching time is required to perform automatic refeeding. Hence time productivity, which is an advantage of a duplex printing device, is poor in this method. Secondly, due to the limitations of the automatic refeed mechanism, only a few sheets can be subjected to duplex printing at once, and when a larger number of duplex-printed sheets is desired, the single duplex printing cycle described above must be repeated. In this case, it is possible to read images on both sides of an original once using an automatic document feeder (ADF), but in order to refeed an original that has been read once and discharged, and read the image printed thereon, an extremely expensive automatic reversing document feeder or recycling document handler (ARDF or RDH) must be installed. Accordingly, the duplex printing cycle cannot be repeated unless expensive image memory is installed.
Fourthly, time is required for perforation and plate loading every time duplex printing is repeated, and although the printing durability of the master (which indicates the number of sheets that can be printed from one perforated master) is sufficient, cost is wasted to re-perforate a new master. Fifthly, when duplex printing is complete, the ink on the printed sheet has not dried, or set, sufficiently, and hence if printing is performed on the reverse side of a printed sheet that has just been printed on the front side, pressing means such as the conveyance roller and press roller are pressed against the image portion such that the printed image is contaminated or disturbed by ink and the like. As a result, sheets which have completed printing on the front side must be separated and conveyed anew, which increases the likelihood of a jam, and moreover, the size of the printing device must be increased, among other problems.
(2) One-Pass Simultaneous Duplex Printing Method Having Two Facing Drums
This method comprises a first print drum, a second print drum disposed opposite the first print drum via a sheet conveyance path, and moving means for bringing the outer peripheral surface of the first drum and the outer peripheral surface of the second drum into and out of contact with each other. By moving the moving means such that the print drums are pressed together, front side printing in which one side, i.e. the front side, of a sheet is printed, and reverse side printing in which the other side, i.e. the reverse side, of the sheet is printed, are performed by passing the sheet only once, or in other words in one pass. Thus a duplex-printed sheet is obtained (see Japanese Unexamined Patent Application Publication H6-71996, for example).
In the duplex printing method described in (1) above, firstly, the two print drums are disposed one above the other and pressed together, and hence the second print drum is required even when simplex printing is performed. Since the two print drums must be pressed together, a perforated master must be wrapped around one of the print drums and a non-perforated master must be wrapped around the other print drum, resulting in the wasteful consumption of a master during simplex printing. Since two master plates are required even during simplex printing, plate costs double in comparison with a typical simplex printing device. Secondly, perforation and plate loading are performed for two plates, and even though perforation may be performed at a higher speed on the non-perforated master, the first print time (FPT) must be slowed.
Thirdly, the inner peripheral surface of a typical print drum is circular, and convex type clampers protruding outward from the outer peripheral surface are provided to hold the master on the outer peripheral surface of the print drums. Hence, either recessed portions must be formed on each print drum in the positions at which the clampers face each other when the two print drums are pressed together to prevent the convex portions of the clampers from interfering with each other, or the two print drums must be separated from each other at these positions. When two printing drums having the same large outer diameter are used, compared with a combination of a print drum and a press roller having a comparatively small outer diameter, the recessed portions must be formed over a wide enough area to enable the clampers to avoid each other, and yet to ensure that the printing area does not become too small, the outer diameter of the print drum must be increased further, thereby creating a vicious circle.
Fourthly, a loud noise is produced when the print drums are brought into contact. Moreover, it is extremely difficult to prevent discharged sheets from becoming wrapped around the print drums, among other problems.
(3) Single Drum Transfer, Single Pressing Means One-Pass Duplex Printing Method
A divided perforated master (duplex printing master) formed with a first perforated image (front side image) and a second perforated image (reverse side image) is wrapped around the outer peripheral surface of a print drum. The reverse side perforated image on the print drum is subjected to intermediate transfer onto a press roller (transfer/pressing means), whereupon printing is performed on the front and reverse sides of a sheet passed therethrough simultaneously (see Japanese Unexamined Patent Application Publication H8-332768, for example). In the duplex printing method of section (3), masters are used efficiently rather than being wasted during simplex printing, and the transfer of wet ink can be suppressed since duplex printing is performed in a single step. As a result, high quality printed images can be obtained. Moreover, the device can be simplified and reduced in size.
However, in the duplex printing method described in (3), firstly, there is a large difference between the density of the front side image, which is produced by transferring one of the first perforated image and second perforated image (front side image and reverse side image) directly onto the sheet from the print drum, and the density of the reverse side image, which is produced by transferring the other perforated image onto the press roller, which has a different ink absorbency to the sheet, and then re-transferring the image onto the sheet. As a result, the image density of the images on the sheet differs between the front side and reverse side.
Secondly, with this method, the sheet feed timing differs greatly between normal simplex printing and duplex printing. As a result, the mechanisms and control system of the sheet feed system become complicated, leading to an increase in cost. Thirdly, the perforated image on the perforated master used during normal stencil printing is a mirror image, and hence in this method, in which intermediate transfer is performed onto a press roller, the perforated image on the transfer side of the master must be created as a normal image. When only mirror images are created, the cost of mirror image processing can be reduced by selecting the master conveyance direction, the orientation of the thermal head, and so on appropriately. In this case, however, where both normal images and mirror images are perforated, a mirror image reversal circuit must be added, leading to an increase in cost.
(4) One-Pass Simultaneous Duplex Printing Method with Two Facing Drums and an Interposed Transfer Cylinder
In this duplex printing method, first and second rotatable print drums comprising ink supply means and having a master wrapped around the outer peripheral surface thereof are used together with a transfer cylinder positioned between the first and second print drums. An ink image is transferred to the transfer cylinder from the second print drum, whereupon the ink image on the transfer cylinder is transferred again onto the reverse side of a sheet, and thus printing is performed simultaneously on the front and reverse sides of the sheet (see Japanese Unexamined Patent Application Publication H8-118774, for example). In the duplex printing method of (4), differences remain in the density and image quality of the printed images on the front side and reverse side, and since a total of three drums, including the transfer cylinder, are disposed in printing device, the size of the device increases. Moreover, cleaning must be performed, and so on.
(5) Duplex Printing Method Comprising a Single Drum, Employing Divided Printing and Simultaneous Reversal
In this duplex printing method, the front and reverse sides of a sheet are printed simultaneously through a single perforation step and a single printing step using a print drum around which a perforated master formed with a first image and a second image is wrapped, sheet supply means for supplying sheets to the vicinity of the print drum, first pressing means for pressing a first side of a sheet supplied by the sheet supply means against the first image on the print drum to form a first printed image on a first side of the sheet, sheet reversing means for reversing the sheet printed with the first printed image on its first side, and second pressing means for pressing a second side of the sheet reversed by the sheet reversing means against the second image on the print drum to form a second printed image on the second side of the sheet (see Japanese Unexamined Patent Application Publication H9-95033, for example).
In the duplex printing method described in (5), by restricting the image size during duplex printing, the printing device can be made more compact than that of the three duplex printing methods described above and the single drum, dividing transfer cylinder, one-pass duplex printing method to be described below. Moreover, there are no obstructions to simplex printing, and no difference in density or the like between the printed images on the front and reverse sides. Duplex printing can be performed at a high speed and in a short amount of time, thus enabling time-saving and high productivity.
However, in the printing device of the duplex printing method described in (5), in the aforementioned Japanese Unexamined Patent Application Publication H9-95033, for example, there remain questions as to the reliability of sheet conveyance and responsiveness to high speed printing. Moreover, the peripheral constitution of the printing portion is complicated, requiring two pressing means (primary and secondary press rollers 17, 24 serving as the first and second pressing means), and two corresponding ink supplying means (7, 8) within the print drum.
(6) Single Drum, Divided Printing, Transfer Cylinder, One-Pass Duplex Printing Method
This is a duplex printing method positioned between the methods described above in (4) and (5) (see Japanese Unexamined Patent Application Publication H10-129100, for example). The basic constitution of a stencil printing device employing this single drum, dividing transfer cylinder, one-pass duplex printing method comprises a print drum having a front side printing area and a reverse side printing area on the outer peripheral surface of the same drum, a transfer cylinder formed on its surface with a reverse side ink image which is transferred by ink passing through the reverse side printing area of the print drum, conveyance means for conveying a sheet between the print drum and the transfer cylinder on which the reverse side ink image is formed, and so on.
In the duplex printing method described in (6), printing is performed on the front and reverse sides of the sheet in a single pass when the sheet passes between the print drum and the transfer cylinder formed with the reverse side ink image. In other words, in this duplex printing method, all that is required is a single print drum, a single ink supply means, a single transfer cylinder, a single perforation unit, and a single plate discharge unit, and hence reductions in the size and cost of the device can be achieved due to the small number of components. Moreover, by setting the diameter ratio of the print drum and transfer cylinder to 2:1, printing can be performed continuously by rotating the print drum and transfer cylinder at a constant rotation speed.
However, in the duplex printing method described in (6), the ease with which ink is released from the outer peripheral surface of the print drum is equal in both the front side printing area and the reverse side printing area, and furthermore, an ink-repellent material is used on the outer peripheral surface layer of the transfer cylinder such that in comparison with a case where printing is performed simultaneously on the front and reverse sides on the basis of an identical perforated image produced from an identical original, for example, the amount of ink transferred from the outer peripheral surface of the print drum to the outer peripheral surface of the transfer cylinder is smaller than the amount of ink transferred directly onto the sheet from the outer peripheral surface of the print drum. As a result, unevenness and differences in the print image density of the front side printed image and the reverse side printed image occur in this duplex printing method also.
(7) A single-step duplex printing device has been proposed which solves all of the problems described above in (1) through (5). In this duplex printing device, simplex printing can be performed without wasting a master, a printed sheet having favorable image quality with no unevenness and differences in the print image density of the front side printed image and the reverse side printed image printed on the sheet can be obtained during duplex printing, and increases in installation space can be suppressed. This novel duplex printing device (see Japanese Unexamined Patent Application Publication 2003-200645 and Japanese Unexamined Patent Application Publication 2003-237207, for example) uses the basic duplex printing method (to be referred to hereafter as “single drum, single pressing means duplex printing method”) described in (5) (employing a press roller as the single pressing means and a single ink supply means), but eliminates the lack of sheet conveyance reliability and high speed printing responsiveness therein.
The duplex printing device disclosed in Japanese Unexamined Patent Application Publication 2003-200645 and Japanese Unexamined Patent Application Publication 2003-237207, proposed by the present applicant and employing the single drum, single pressing means duplex printing method described above has a basic constitution comprising a printing portion having a print drum around which a divided perforated master formed with a first image and a second image side by side (within the length of a single plate (the length of the master corresponding to the circumference of the print drum, likewise hereafter) is wrapped in the rotary direction of the print drum, and pressing means selectively which can be brought into or out of contact with the print drum, a sheet feeding portion for feeding a sheet toward the printing portion, a sheet discharging portion for discharging a printed sheet printed in the printing portion, an auxiliary tray (reefed storage means) for temporarily storing the printed sheet printed with a printed image on its front side in the printing portion, refeeding means for reversing the front-side printed sheet temporarily stored in the auxiliary tray and refeeding the sheet toward the printing portion, and a switching member (switching means) for guiding the sheet coming out of the printing portion toward the reefed storage means or the sheet discharging portion.
During duplex printing, a first sheet is fed from the sheet feeding portion to the printing portion, where either one of the first image and second image is printed on its front side, and then the switching member guides the printed first sheet toward the auxiliary tray. Next, a second sheet is fed from the sheet feeding portion to the printing portion and printed with either one of the first image and second image on its front side, while the first sheet is refed by the sheet refeeding means to the printing portion, where the other of the first image and second image is printed on the reverse side thereof. The switching member then guides the first sheet toward the sheet discharging portion, and guides the second sheet toward the auxiliary tray. Thus both sides of the sheet can be printed in a single rotation of the print drum, excluding the first and last sheet passage.
In the duplex printing device disclosed in Japanese Unexamined Patent Application Publication 2003-237207, during duplex printing, a first image is perforated from a first position at a predetermined remove from the leading end of a master, irrespective of the size of the sheet, and a second image is perforated from a second position at a predetermined remove from the first position such that a gap (margin portion) is provided between the first image and second image. By provided an appropriate margin portion, deviation in the image positions on the front and reverse sides of the sheet can be prevented. Reading information about the original image is stored in an image memory and recalled during perforation. According to this duplex printing device, deviation in the image positions on the front and reverse sides of the sheet can be prevented, time productivity during duplex printing can be improved, and simplex printing can be implemented without waste.
In the duplex printing device (1) disclosed in Japanese Unexamined Patent Application Publication 2003-200645, as illustrated in FIG. 7 and described in paragraph [0008], the following keys (buttons) and display devices are provided on an operating panel (103): a duplex print key (117) for selecting and setting a duplex print mode for performing duplex printing; a simplex print key (118) for selecting and setting a simplex print mode for performing simplex printing; a perforation start key (104) also referred to as a start key for activating and setting a series of operations comprising plate discharging, original image reading, plate perforation and wrapping, and printing; a print start key (105) also referred to as a print key for activating and setting a normal duplex printing operation or simplex printing operation; and so on. When the duplex print key (117) is depressed, an LED (117a) in the vicinity of the duplex print key (117) is illuminated, and when the simplex print key 118 is depressed, an LED (118a) in the vicinity of the simplex print key (117) is illuminated, thereby showing the operator that the duplex print mode or the simplex print mode has been set. The duplex printing device (1) illuminates the LED (118a) to indicate that the simplex mode is set in a preset initial state (also referred to as “initial set mode”).
However, the duplex printing method described in (7) has the following problems. Firstly, if a mistake is made in setting the duplex print mode for performing duplex printing and the simplex print mode for performing simplex printing, then a master is wasted during perforation.
This problem occurs as follows. In the duplex printing device (1) of Japanese Unexamined Patent Application Publication 2003-200645 and Japanese Unexamined Patent Application Publication 2003-237207, as described above, the LED (118a) is illuminated in the initial set mode of the duplex printing device (1) to show the operator that the simplex print mode is set. However, in cases such as when the operator is in a hurry to perform duplex printing, for example, s/he may press the print start key (105) without noticing that the LED 118a indicating the simplex print mode is illuminated, as a result of which simplex printing is performed wastefully in the simplex print mode. Alternatively, s/he may press the perforation start key (104) with the intention of activating perforation for duplex printing, as a result of which perforation is performed for simplex printing mistakenly in the simplex print mode, causing an expensive master to be wasted.
This suggests that it is desirable for the operator or user (to be referred to as “user” hereafter) to be able to select and set either the duplex print mode or the simplex print mode freely as the initial set mode (default), which is set in advance when the power of the duplex printing device (1) is switched on or during mode clearance to clear the various modes executed by the duplex printing device (1), depending on the frequency with which the user performs duplex printing or simplex printing. It also suggests that the user is not given sufficient warning by simple display means such as the LED (118a).
Secondly, the maximum sheet size of the sheets used during duplex printing in the duplex printing device (1) of the two aforementioned publications, comprising a plate cylinder (print drum) (12) which is capable of performing simplex printing on A3 size sheets, for example, is A4 portrait (described as A4 “landscape” in Japanese Unexamined Patent Application Publication 2003-200645, but described hereafter as A4 “portrait” in a direction seen from a user who is facing the operating panel (103)). However, when duplex printing is performed on sheets having a greater size than A4 portrait (also referred to simply as “A4” hereafter), sheet jams occur. This also occurs in a duplex printing device having bank feeding means for selectively feeding sheets of a plurality of sizes stacked on a plurality of feed tables.
Thirdly, the duplex printing device (1) of the two aforementioned publications comprises a sheet thickness setting key (116) for selecting and setting a sheet thickness (one of either “normal”, “thin”, or “thick”, for example). However, if “normal” is selected and set as the sheet thickness by depressing the key (116) mistakenly when “thick” was intended, again a sheet conveyance jam occurs.
Technologies relating to the present invention are also disclosed in, e.g. Japanese Unexamined Patent Application 2003-312914.